1. Field of the Invention
This invention relates to an improved full range hydromechanical transmission and more particularly to an improved full range hydromechanical transmission for agricultural and industrial type tractors.
2. Description of the Prior Art
Hydromechanical transmissions are transmissions which combine mechanical gearing with a hydrostatic arrangement. Although mechanical transmissions are generally more efficient and reliable than pure hydrostatic transmission, they have the disadvantage of not being infinitely variable as are the more expensive hydrostatic transmissions. Likewise, the hydrostatic transmissions have a major disadvantage in that they generally require larger components, such as larger pumps and motors, as the transmission increases in maximum capacity.
In order to satisfy space limitations, reduce cost, increase efficiency and provide an infinitely variable speed transmission which is capable of supplying power over a full spectrum of forward and reverse speeds, transmission designers have combined the best features of both transmissions and have built the so-called hydromechanical transmission. However, each hydromechanical transmission has been designed for a particular purpose and their use as a tractor transmission is somewhat limited.
Three relevant U.S. patents on hydromechanical transmissions include U.S. Pat. No. 3,534,632, issued to Smith in 1970; U.S. Pat. No. Re. 27,307, issued to DeLalio in 1972; and U.S. Pat. No. 3,736,813, issued to Kress et al in 1973. All three patents differ from the present invention in at least one noticeable way. Specifically, the Smith transmission utilizes a plurality of gear sets to provide a synchronous speed shifting transmission. The present invention simplifies the mechanical gearing by using only one compound planetary. The second patent, issued to DeLalio, claims a transmission having a full hydrostatic low range and a hydromechanical high range. Such a transmission is unsatisfactory for applications in tractors because the major working speeds on a tractor are in the range of about 3-7.5 mph. This range would correspond to a range of output/input ratios of about 0.25-0.56 rpm, most of which would be in the inefficient full hydrostatic operating range. Lastly, the Kress patent is a limited range variable speed transmission utilizing a collar shifted portion to extend the limited ranges. The present invention eliminates the necessity of having such a collar shift portion.